Flat 1x4 Wood Toprail Refinish Help

I live in New Hampshire. The deck is Trex but the top rail is wood. The flat top rail of my deck is 1x4 wood (maple?). Two years ago it was put in and without realizing Poly was not good for outdoor surfaces (or so I am informed because it blisters and peels which is what happened over the two years) I had put two beautiful coats of poly on it. Now to get rid of the areas where is has peeled (mostly near the joints over the deck rail support posts) I have sanded lightly with an orbital sander to get the sheen off and get rid of the peeling, etc. It's pretty flat and smooth now but what can I use to put on the top rail now to protect it better? Do I have to sand it to bare wood to make sure all the old poly is gone? Should I just stick with more poly, or is there something else I can/should put over the top of the sanded poly that's already there? The deck is half in full sun, and half in part sun (it's L-shaped around the back and side of the house.